In our previous application WO 93/10910 we describe a device for projecting droplets from a nozzle that is excited to project liquid therefrom.
In our previous application WO99/54140 we describe a device and method for projecting liquid as jets or droplets from multiple nozzles formed in a material layer. The nozzles are formed in a transducer that incorporates a finger with liquid being supplied to an inner end of the nozzles. By continuously stimulating excitation of the finger motion at a certain frequency, the nozzle will eject a continuous droplet stream from an outer end of the nozzle.
In the type of device described in WO 93/10910 and WO 99/54140, there is no local chamber and hence no reflected pressure waves of the type found in many other known devices. The invention, as described later, is equally applicable to the devices in either of these documents and indeed to any open-reservoir structure with front-face actuators.
During printing or fluid dispensing from the type of devices described in WO99/54140 and WO 93/10910, debris, dried ink, solute or other unwanted materials (“crud”) can form on the material layer, both on the outlet side and the inlet side. It is necessary to periodically clean and remove this debris and this is particularly important when printing biological material (e.g. for diagnostic purposes) or fluids that evaporate easily. With this type of medium, the nozzles through which the material is being ejected get crusty when not in use, even if that use is only for a relatively short period of time.
In U.S. Pat. No. 5,543,827, U.S. Pat. No. 6,267,464 and U.S. Pat. No. 6,196,656 devices are described that contain a number of dedicated piezoelectric actuators that can be operated to result in ultrasonic actuation of the material layer to loosen debris on the material layer. Although this can be an effective way of clearing the debris, the requirement of piezoelectric actuators solely for the use of cleaning increases the cost and complexity of such devices.
U.S. Pat. No. 5,329,293 describes a technique for clearing ink jet heads which uses sub-firing signals in between ejections which are ineffective in causing ejection, and where the sub-firing signals are synchronous with the firing signals, i.e. the sub-firing signal is at a predetermined time after the firing signal. This is applied in the context of standard ink jet heads where the actuators are situated within or so as to form enclosed chambers.
The use of sub-firing signals on their own enable the fluid meniscus to re-suspend or re-dissolve the crud deposited on the outer face of the nozzle as a result of solvent evaporation (or carrier evaporation if it's a suspension), as the meniscus is caused to move back and forth. However, even with such an approach, there is a concentration of the crud in the bulk fluid near the inner face of the nozzle.